1. Field of the Invention
The present invention relates to a device for manufacturing a semiconductor device having a circuit constituted by a thin film and, or example, to a device for manufacturing an electro-optical device typified by a liquid display device and an electric device having the electro-optical device as a part. In this connection, in the present specification, a semiconductor device designates in general a device capable of functioning by the use of semiconductor characteristics and includes the above electro-optical device and electric device.
2. Description of the Related Art
In recent years, research and development have been widely conducted on the technologies for performing a laser annealing processing to an amorphous semiconductor film or a crystalline semiconductor film (semiconductor film which is not a single crystal but a polycrystal or a micro-crystal), that is, non-single crystal semiconductor film formed on an insulating substrate such as a glass substrate or the like to crystallize the non-single crystal semiconductor film or to improve its crystallinity. A silicon film is often used as the above semiconductor film.
A glass substrate has advantages that it is cheap and has good workability and is easy to make a large area substrate in comparison with a quartz substrate which has been conventionally used. This is because the above research and development have been carried out. Also, it is because the melting point of the glass substrate is low that a laser is widely used for crystallizing the semiconductor film. The laser can apply high energy only to a non-single crystal film without increasing the temperature of the substrate too much.
The crystalline silicon film is called a polycrystalline silicon film or a polycrystalline semiconductor film because it is made of many crystal grains. Since the crystalline silicon film subjected to a laser annealing processing has high mobility, a thin film transistor (hereinafter referred to as TFT) is formed by the use of the crystalline silicon film and, for example, is widely used for a monolithic liquid crystal electro-optical device having a glass substrate and TFTs for driving a pixel and for a driving circuit.
Also, a laser annealing method of transforming the high-power laser beam of a pulse oscillation such as an excimer laser into a square spot several cm square or a linear beam 10 cm or more in length at an irradiate surface by the use of an optical system and of scanning a semiconductor film with the laser beam (or moving a spot irradiated with the laser beam relatively to an irradiate surface) has been widely used because it increases mass productivity and is excellent in an industrial view point.
In particular, when a linear laser beam is used, the whole irradiate surface is irradiated with the linear laser beam only by scanning the irradiate surface in the direction perpendicular to the direction of the line of the linear laser beam, which therefore produces high mass productivity. In contrast to this, when a spot-like laser beam is used, the irradiate surface needs to be scanned with the laser beam in the back-and-forth direction and in the right-and-left direction. The irradiate surface is scanned with the linear laser beam in the direction perpendicular to the direction of the line of the linear laser beam because the direction is the most efficient scanning direction. The method of using the linear laser beam into which the laser beam emitted from the excimer laser of pulse oscillation is transformed by the use of a suitable optical system for the laser annealing processing has become a mainstream technology.
In FIG. 1 is shown an example of the constitution of an optical system for transforming the cross section of the laser beam into a linear shape at an irradiate surface. This constitution is extremely ordinary and all the above optical systems are similar to FIG. 1. This constitution not only transforms the cross section of the laser beam into the linear shape but also homogenizes the energy of the laser beam at the irradiate surface. In general, an optical system homogenizing the energy of the beam is called a beam homogenizer.
In the case where an excimer laser which is ultraviolet radiation is used as a light source, it is recommended that the base material of the above optical system be quartz because the quartz can produce a high transmittance. Also, it is recommended to use a coating capable of producing a transmittance of 99% or more to the wavelength of the excimer laser.
First, a side view in FIG. 1 will be described. A laser beam emitted by a laser oscillator 101 is divided into the direction orthogonal to the direction of travel of the laser beam by cylindrical lens arrays 102a and 102b. The applicable direction is called a vertical direction in the present specification. When a mirror is arranged in the middle of the optical system, the above vertical direction is bent in the direction of the light bent by the mirror. In this constitution, the laser beam is divided into four portions. These divided laser beams are once unified to one laser beam by a cylindrical lens 104. The unified laser beam is reflected by a mirror 107 and then is again focused on one laser beam at an irradiate surface 109 by a doublet cylindrical lens 108. The doublet cylindrical lens means the one constituted by two cylindrical lenses. The doublet lens homogenizes the energy in the width direction of the linear laser beam and determines a length in the width direction of the laser beam.
Next, a top view will be described. The laser beam emitted by the laser oscillator 101 is divided by cylindrical lens arrays 103 into the direction orthogonal to the direction of travel of the laser beam and in the direction orthogonal to the vertical direction. The applicable direction is called a lateral direction in the present specification. When a mirror is arranged in the middle of the optical system, the above lateral direction is bent in the direction of the light bent by the mirror. In this constitution, the laser beam is divided into seven portions. These divided laser beams are once converged on one laser beam at the irradiate surface 109 by the cylindrical lens 105. This homogenizes the energy in the length direction of the linear laser beam and determines the length of the linear laser beam.
The above lenses are made of synthetic quartz to respond to the excimer laser. Also, their surfaces are coated such that they well transmit the excimer laser, whereby the transmittance of one lens to the excimer laser is made 99% or more.
The linear laser beam transformed by the above constitution is applied to the non-single crystal silicon film while it is gradually shifted and superposed in the direction of the width of the linear laser beam to subject the whole surface of the non-single crystal silicon film to laser annealing to thereby crystallize the non-single crystal silicon film or to improve the crystallinity thereof.
Next, a typical method of forming a semiconductor film to be irradiated with the laser beam will be described.
First, a Corning 1737 substrate 0.7 mm thick and 5 inch square was prepared as a substrate. A SiO2 film (silicon oxide film) having a thickness of 200 nm was formed on the substrate with a plasma CVD device and the amorphous silicon film (hereinafter referred to as “a-Si film”) having a thickness of 50 nm was formed on the surface of the SiO2 film.
The substrate was heated at 500° C. in a nitrogen atmosphere for 1 hour to reduce the concentration of hydrogen in the film, whereby the resistance to laser of the film was remarkably improved.
A XeCl excimer laser L3308 (wavelength=308 nm, pulse width=30 ns) made by Ramda Corp. was used as a laser device. The laser device generates a pulse oscillation laser and has a capacity producing an energy of 500 mJ/pulse. The size of the laser beam is 10-30 mm (both in full width at half maximum) at the exit of the laser beam. The exit of the laser beam is defined, in the present specification, as a plane perpendicular to the direction of travel of the laser beam right after the laser beam is emitted by the laser irradiation device.
In general, the shape of the laser beam generated by the excimer laser is rectangular and ranges from 3 to 5 when expressed in aspect ratio, and as the position is nearer to the center of the laser beam, the intensity of the laser beam is stronger, that is, the intensity of the laser beam shows a Gaussian distribution. The size of the above laser beam was transformed into a linear laser beam of 125 mm×0.4 mm having a uniform energy distribution by an optical system having a constitution shown in FIG. 1.
According to the experiment of the present inventor, it was found that the pitch of superposition of 1/10 times the width (full width at half maximum) of the linear laser beam was most suitable in the case where the above semiconductor film was irradiated with the linear laser beam. This improved homogeneity in crystallinity in the film. In the above example, the above full width at half maximum was 0.4 mm and hence the semiconductor film was scanned and irradiated with the laser beam of the excimer laser at a pulse frequency of 30 Hz, at a scanning speed of 1.0 mm/sec, at an energy density of 420 mJ/cm2 at the surface irradiated with the laser beam. The above-described method is an extremely ordinary method used for crystallizing a semiconductor film by the use of the linear laser beam.
When the silicon film annealed with the above linear laser beam was very carefully observed, very weak interference fringes were observed. This is because when the divided laser beams were again converged on one region, the divided laser beams interfered with each other. However, since the excimer laser has a coherence length ranging from about several micron to several tens micron, it does not produce strong interference.
The state of the art in the excimer laser can oscillate high-power, high-repetition pulses (about 300 Hz) and hence is widely used for crystallizing the semiconductor film. When the liquid crystal display using a low-temperature polysilicon TFT, which has been brought to a commercial stage in recent years, was manufactured, the excimer laser is widely used in the crystallization process of the semiconductor film.
In recent years, the maximum power of a YAG laser has been remarkably increased. Since the YAG laser is a solid state laser, it is easy to handle and maintain as compared with the excimer laser which is a gas laser. The present inventor considered the possibility of the YAG laser being used for crystallizing the semiconductor film in consideration of the increasing power of the YAG laser.
It is well known that the YAG laser emits a laser beam having a wavelength of 1065 nm as a fundamental wave. The absorption coefficient of the silicon film to the laser beam is very low and hence can not be used in this state for crystallizing the a-Si film which is one of the silicon films. However, the laser beam can be converted into the laser beams having shorter wavelengths by the use of the non-linear optical crystal. The converted laser beams are called the second harmonic (533 nm), the third harmonic (355 nm), the fourth harmonic (266 nm), and the fifth harmonic (213 nm), depending on the converted wavelength.
Since the second harmonic has a wavelength of 533 nm and has a sufficient absorption coefficient to the a-Si film, it can be used for crystallizing the a-Si film. However, its absorption coefficient to the a-Si film is not so high as that of the excimer laser. The third harmonic, the fourth harmonic, and the fifth harmonic are very high in the absorption coefficient to the a-Si film and hence can crystallize the semiconductor film with a high degree of energy efficiency.
The maximum power of the third harmonic of the state-of-the-art YAG laser is about 750 mJ/pulse. Also, the maximum power of the fourth harmonic is about 200 mJ/pulse. The maximum power of the fifth harmonic is lower than the above maximum power and hence the fifth harmonic is not suitable for crystallizing the semiconductor film. From the viewpoint of both the power and the absorption coefficient to the a-Si film of the laser beam, it is best at the present time to use the second harmonic or the third harmonic.
Next, in the case where the YAG laser is used for crystallizing the semiconductor film, it is preferable for mass production that the shape of the laser beam at the irradiate surface is linear. It is preferable that the above optical system is applied to the YAG laser as it is. This possibility will be considered in the following.
First, the difference between the YAG laser and the excimer laser will be described. The shape of the laser beam emitted by the excimer laser is generally rectangular and the shape of the laser beam emitted by the excimer laser is generally circular. The dominating size of the laser beam having large power exceeding 500 mJ/pulse and high repetition over 200 Hz is about 10-30 mm and the above optical system is tailored to the size of the laser beam. On the other hand, the size of the laser beam of the YAG laser over 500 mJ/pulse is a circle having a diameter of about 10 mm. In order to tailor the YAG laser 10 mm in diameter to the above optical system, it is recommended that the circular laser beam be transformed into an ellipsoidal one by the use of the beam expander capable of changing the size of the laser beam. In this case, it is recommended that the above circular laser beam be elongated by three times to an ellipsoidal laser beam 30 mm in long diameter and 10 mm in short diameter by the use of the beam expander constituted by cylindrical lenses capable of elongating the size of the laser beam in one direction.
An example of an optical system in which the above beam expander is built in the optical system shown in FIG. 1 to be adapted to the YAG laser 300 will be shown in FIG. 3. FIG. 3 shows only a top view. In FIG. 3 and FIG. 1, the same reference numerals designate the lenses having the same shape.
A cylindrical lens 301 has a focal length of 100 mm, a length and a width of 50 mm, and a thickness of 10 mm. A laser beam enters the cylindrical lens 301. A cylindrical lens 302 has a focal length of 200 mm, a length and a width of 50 mm, and a thickness of 10 mm. These lenses are arranged at a distance of 400 mm from each other. This elongates the laser beam three times in one direction.
Next, the difference in coherence length between the YAG laser and the excimer laser will be described. As described above, the excimer laser has a coherence length of about several micron to several tens micron and hence produces a very weak optical interference when the laser beam emitted by the excimer laser passes through an optical system for dividing the laser beam and then converging it on one point. On the other hand, the YAG laser has a very long coherence length of 1 cm and hence the effect of interference produced by the YAG laser is not negligible.
If the laser beam emitted by the YAG laser is passed through the optical system shown in FIG. 3 to be transformed into a linear laser beam 200, the linear laser beam 200 has an energy distribution in which energy is repeatedly increased or decreased like a grid pattern.
The energy distribution shaped like a grid pattern is produced by an optical interference. In FIG. 2A, dark lines 201 designate regions having comparatively high energy and blank lines 202 between the dark lines 201 designate regions having comparatively low energy.
If the a-Si film is crystallized with the linear laser beam 200 having the energy distribution shaped like a grid pattern, the a-Si film is heterogeneously crystallized. FIG. 2B shows the surface of a silicon film 203 crystallized with the linear laser beam. As described above, since the a-Si film is irradiated with the linear laser beam while the linear laser beam is shifted and superposed by 1/10 times in the width direction of the laser beam, interference fringes parallel to the line direction of the linear laser beam cancel each other to become light, but interference fringes 204, 205 parallel to the width direction of the linear laser beam remain strongly dark. In FIG. 2B, dark lines 204 designate regions having comparatively high energy and blank lines 205 between the dark lines 204 designate regions having comparatively low energy.